Rapid spread of a communication terminal with a wireless local area network (LAN) (also referred to as Wi-Fi [registered trademark] or wireless fidelity) function, such as a smart phone and a tablet-type communication terminal, has recently led to widespread use of a wireless LAN. There are wireless LAN related standards including widely prevalent IEEE 802.11a, b, g, and n, specifications of which being developed by the Institute of Electrical and Electronics Engineers, Inc. (IEEE).
In wireless LAN standards such as IEEE 802.11a, b, g, and n, an operation for detecting frame collision is not defined. Accordingly, a wireless LAN communication terminal uses an access control method called a carrier sense multiple access with collision avoidance (CSMA/CA) method in a media access control (MAC) layer. A communication terminal connected to a wireless LAN receives a signal in a transmission medium (wireless frequency) to be used and checks (carrier sense) whether or not another communication terminal currently in communication exists before starting data transmission.
When determining that another communication terminal currently in communication using a wireless frequency band to be used does not exist and communication is feasible, each communication terminal connected to the wireless LAN avoids collision (collision avoidance) of transmitted frames (data) by starting communication (multiple access) after a waiting time corresponding to a generated random number elapses.
In this case, when random numbers generated by the respective communication terminals have mutually close values, data transmission timings become close to one another and data collision occurs. While each communication terminal checks receipt of data by receiving a positive acknowledgement (ACK), an ACK is not transmitted when such collision occurs. Consequently, each communication terminal, not receiving an ACK when a predetermined time elapses after transmitting data, generates a random number again and retransmits the data after a waiting time corresponding to the random number elapses.
PTL 1 describes a method of avoiding collision in wireless communication by receiving a state notification signal indicating an operating state and an operation timing of a neighboring communication terminal, and controlling a time slot width, a communication timing, and the like of the own communication terminal in accordance with communication status of the neighboring communication terminal.
Further, PTL 2 describes a method of decreasing a packet collision probability by randomly delaying data transmission.
NPL 1 discloses a method of, when communication terminals move and approach one another, exchanging information indicating stored contents and synchronizing the stored contents, in accordance with an epidemic method. FIG. 21 is an explanatory diagram illustrating an example of a communication system synchronizing stored contents by an epidemic method. As illustrated in FIG. 21, communication terminals 1 and 2 cyclically transmit and receive a control message including a summary vector being a list of content stored locally to and from adjacent separate communication terminals 2 and 1, and exchange the summary vectors. Then, each of the communication terminals 1 and 2 detects difference between content stored in the other communication terminal 2 or 1 being a source of a control message, and content stored in the own terminal 1 or 2, in accordance with a summary vector received from the other communication terminal 2 or 1. In the example illustrated in FIG. 21, the communication terminal 1 stores content A, content B, and content C, and the communication terminal 2 stores the content A and the content B. Accordingly, difference between the content stored in the communication terminal 1 and the content stored in the communication terminal 2 is the content C. In order to eliminate the difference, the communication terminal 2 requests transmission of the content C to the communication terminal 1 being the source of the control message. In response to the request, the communication terminal 1 transmits the content C to the communication terminal 2. By the aforementioned processing, content difference is transmitted and received between the communication terminal 1 and the communication system 2 in the communication system illustrated in FIG. 21. Then, content stored in each of the communication terminals 1 and 2 becomes identical to each other, and the stored contents are synchronized.